1. Field of the Invention
The invention relates to the field of data recording on optical discs, and more particularly to methods of locating a portion of a track identified by an address.
Data are recorded in tracks which are concentric circles, or in spiral tracks where the track pitch is very small compared with the track radius so that the track may be considered a circle for addressing purposes. Typically each track itself is divided into segments or sectors, and each of these is in turn divided into frames. The frame is really the basic unit for data retrieval, because it frame is essentially self-contained: it generally begins with a "servo" field which contains control marks or pre-formatted pits defining the track centerline, clock and frame synchronizing information, and frame address information.
Because of the large number of frames which may be recorded on a disc, a significant portion of the total space may be occupied by addresses. Because these addresses become longer as more frames are squeezed onto a disc, locating a particular position as an optical head moves across the disc becomes increasingly difficult.
2. Description of the Prior Art
U.S. Pat. No. 5,126,991 discloses a technique for speeding identification of a location on the disc by using a "creeping" code for the address. With this technique successive addresses are represented by respective code words differing in one bit position only. This improves the reliability of identifying the address when the scanning beam is not locked onto the center of a particular track.
As taught in the '991 patent, to speed identification and correction of errors, track addresses may be repeated in different locations along the track. Accuracy may be further improved by multiplexing the address: it is split into parts, and so-called servo areas of individual frames contain only one of these parts.
A particular improved technique for identifying locations on a disc is disclosed in ANSI document X3B11/91/300-043, dated Apr. 29, 1991. The MASS FORMAT described therein uses address nibbles which are pre-formatted in servo areas of successive frames. When a creeping code is used, the part of the address which is incrementing can be used as a segment address. To do this it is necessary to establish segment synchronization, by reading and decoding enough address nibbles that the respective frame locations in the segment can be known; from this information the position of the beginning of the segment can then be determined. After this synchronization, a microprocessor can then determine where the other address bytes in the sequence are located.